Mammalian brains function more at the level of ensembles of neurons
[E]volution has built the mammalian brain by elaborating on basic plans already present in our distant ancestors. In simpler organisms it is possible to identify not just types of cells, but individual neurons – in nematodes, the 302 neurons have all been named. In insects, you can see the equivalent individual neurons repeated in each segment of the ventral nerve cord. Those nervous systems function based on the actions of individual neurons and their interconnections. Mammalian brains function more at the level of ensembles of neurons, but the basic logic is similar – evolution has built these brains by expanding the numbers of cells of ancestral types, so that what was once a single neuron is now a population of neurons of the same “type”. Evolution has also increased the diversity of subtypes, which are deployed and combined in myriad ways to generate the incredibly complex circuitry we seek to understand.
That is the reason I argue that cell types are the fundamental units of the nervous system [...].
Quoted on Mon Sep 2nd, 2013